Suicide bombers driving two large vehicle bombs again penetrated the heart of Baghdad today, killing 147 people and wounding more than 700, less than three months before a national poll that will be contested on supposed security gains.

The attacks had been both feared and anticipated after an almost identical strike destroyed the finance and foreign ministries, in roughly the same area, just over two months ago. Sunday's explosions were blamed on Sunni militants, who had claimed responsibility for the devastating summer explosions and pledged to wreak further havoc in a bid to destabilise the fragile government ahead of a national election planned for 16 January.

One truck bomb, just before 10.30am, destroyed the justice ministry, housing many of the city's judges, lawyers and court rooms, while minutes later a second badly damaged the Baghdad governorate, around 300 metres away.

Frequent displays of violence over five years have conditioned Baghdadis to savagery, but repeated scenes of desperate and dying employees hanging from crumpled government buildings appear to have struck a fresh nerve and further dented public confidence in the security forces. The death toll of the August bombs was 132, according to health officials, with just under 500 injured, meaning at least 1,200 public servants have been killed, or maimed by recent attacks.

"I saw bodies sprawled over the broken panes of the justice ministry," said Mohammed Falah, 30, from the central suburb of Haifa Street, whose newly renovated sandwich shop across the road was laid waste. "They were mostly women and the guards stopped us from helping them carry the bodies out of the ruins in blankets. They were carrying four or five at a time. When I regained consciousness there was a woman's leg next to me in my shop. I picked it up and gave it to the ambulance."

A firefighter sitting nearby said he had carried 15 victims from the ravaged building. "Only one was alive," he said.

Next to him a man in a blood-soaked shirt, whose daughter was feared dead, sobbed uncontrollably as he blamed security lapses. "After the foreign ministry, they had promised to protect us," he said. "What message does it send when a government can't protect its own people in its own buildings?"

Exposing the vulnerability of institutions has been a stated goal of al-Qaida militants whom the prime minister, Nour al-Maliki, again insisted are backed by figures from the regime of executed former president Saddam Hussein. Extra restrictions had been placed on traffic after the August bombs, limiting lorries to maximum loads of one tonne throughout Baghdad before 4pm. Investigators were trying to establish how the bombers again slipped a security dragnet in the heart of the capital that makes extensive use of explosive detection wands at numerous checkpoints.

Huge craters filled with water from burst mains marked ground zero of each explosion. Despite being ringed by nine-foot high blast walls, the closest corner of the justice ministry to the detonation point was a mere three metres away. The blast had ricocheted across the road to a public works building, setting it alight and killing at least five people. Security cameras on the roof of the public works building revealed that the justice ministry bomb was carried by a truck – a fact that if confirmed is sure to intensify criticism of security officials who seem unable, or unwilling, to impose their will.

In a bid to take control of the deteriorating security situation, Maliki inspected the devastation late today and vowed to find the conspirators. He again blamed Syrian leaders for harbouring Ba'athist figures who he alleged were directing a subversive campaign.

Both countries have remained at loggerheads since August after Maliki partly blamed Syria for facilitating the summer blasts. Syria and Iraq recalled their Ambassadors after the row and Damascus denied playing any role in the carnage.